It has been known heretofore that ceria-based oxides are used as a co-catalyst of a three-way catalyst for treatment of exhaust gas discharged from automobiles. Here, the three-way catalyst is a catalyst having a function to remove carbon monoxide, hydrocarbons and nitrogen oxides from the exhaust gas generated with combustion of an internal combustion engine.
In view of recent tendencies toward higher performance and high-temperature use of the catalyst, there is a demand to develop a catalyst that has a high specific surface area and high heat resistance enough to resist decrease in the specific surface area even at high temperatures.
A solid solution comprising ceria, zirconium and an oxide of a rare earth element (lanthanum or the like) other than cerium is recently proposed as one of materials showing such properties (e.g., cf. Claim 6 and examples in Patent Document 1, Claim 2 and examples in Patent Document 2, and Claim 2 and examples in Patent Document 3).
A usually used process for producing such a solid solution is a solution method such as a coprecipitation method. Production of the solid solution by the coprecipitation method is normally so conducted that a solution containing cerium ions, zirconium ions and rare earth ions is prepared, a pH of the solution is controlled to obtain precipitates (coprecipitates) containing cerium, zirconium and the rare earth element, and the coprecipitates are subjected to a heat treatment to convert to oxides.
Patent Document 1: JP-A-2000-319019
Patent Document 2: JP-A-11-292538
Patent Document 3: JP-A-11-292539